Field of the Invention
This invention relates polymeric medical devices, in particular, bioresorbable stents or stent scaffoldings
Description of the State of the Art
This invention relates to radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of a scaffold or scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it possibly physically holds open and, if desired, expands the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site.
Delivery includes inserting the stent through small lumens using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited than for those lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy uses medicated stents to locally administer a therapeutic substance. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffold with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolds may also serve as a carrier of an active agent or drug. An active agent or drug may also be included on a scaffold without being incorporated into a polymeric carrier.
Stents are generally made to withstand the structural loads, namely radial compressive forces, imposed on the scaffold as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength if its function is to support a vessel at an increased diameter. Radial strength, which is the ability of a stent to resist radial compressive forces, relates to a stent's radial yield strength and radial stiffness around a circumferential direction of the stent. A stent's “radial yield strength” or “radial strength” (for purposes of this application) may be understood as the compressive loading or pressure, which if exceeded, creates a yield stress condition resulting in the stent diameter not returning to its unloaded diameter, i.e., there is irrecoverable deformation of the stent. See, T. W. Duerig et al., Min Invas Ther & Allied Technol 2000: 9(3/4) 235-246. Stiffness is a measure of the elastic response of a device to an applied load and thus will reflect the effectiveness of the stent in resisting diameter loss due to vessel recoil and other mechanical events. Radial stiffness can be defined for a tubular device such as stent as the hoop force per unit length (of the device) required to elastically change its diameter. The inverse or reciprocal of radial stiffness may be referred to as the compliance. See, T. W. Duerig et al., Min Invas Ther & Allied Technol 2000: 9(3/4) 235-246.
When the radial yield strength is exceeded, the stent is expected to yield more severely and only a minimal force is required to cause major deformation. Radial strength is measured either by applying a compressive load to a stent between flat plates or by applying an inwardly-directed radial load to the stent.
Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. For example, a radially directed force may tend to cause a stent to recoil inward. In addition, the stent must possess sufficient flexibility to allow for crimping, expansion, and cyclic loading.
Some treatments with stents require its presence for only a limited period of time. Once treatment is complete, which may include structural tissue support and/or drug delivery, it may be desirable for the stent to be removed or disappear from the treatment location. One way of having a stent disappear may be by fabricating a stent in whole or in part from materials that erode or disintegrate through exposure to conditions within the body. Stents fabricated from biodegradable, bioabsorbable, bioresorbable, and/or bioerodable materials such as bioabsorbable polymers can be designed to completely erode only after the clinical need for them has ended.
In addition to high radial strength, a vascular scaffold must have sufficient resistance to fracture or sufficient toughness. A vascular scaffold is subjected to a large deformation during use, in particular, when it is crimped to a delivery diameter and when it is deployed. A scaffold may be susceptible to fracture when in use which can negatively impact performance and even lead to device failure. Fabricating a polymer-based scaffold that has sufficiently high radial strength as well as resistance to fracture is a challenge.
It is advantageous for vascular scaffolds to have thin struts while maintaining adequate radial strength. Thin struts lead to a lower profile device in the crimped state for better deliverability. After implantation, neointima proliferates until stent struts are covered. Consequently, thinner struts have less neointimal formation and less area obstruction of the vessel. Lastly, thin struts disturb blood flow less and are less thrombogenic. However, polymer based materials can be orders of magnitude lower in strength in terms of ultimate strength and stiffness compared to metallic alloys. Fabricating a polymer-based scaffold that has sufficiently high radial strength at strut thicknesses comparable to current metallic stents is therefore a challenge.
Additionally, treating peripheral vascular disease percutaneously in the lower limbs is a challenge with current technologies. Long term results are sub-optimal due to chronic injury caused by the constant motions of the vessel and the implant as part of everyday life situations. To reduce the chronic injury, a bioresorbable scaffold for the superficial femoral artery (SFA) and/or the popliteal artery can be used so that the scaffold disappears before it causes any significant long term damage. However, one of the challenges with the development of a femoral scaffold and especially a longer length scaffold (4-25 cm) to be exposed to the distal femoral artery and potentially the popliteal artery is the presence of fatigue motions that may lead to chronic recoil and strut fractures especially in the superficial femoral artery, prior to the intended bioresorption time especially when implanted in the superficial femoral artery.
Fabricating a polymer-based scaffold for treating the SFA is even more challenging than for coronary applications. A scaffold in the SFA and/or the popliteal artery is subjected to various non-pulsatile forces, such as radial compression, torsion, flexion, and axial extension and compression. These forces place a high demand on the scaffold mechanical performance and can make the scaffold more susceptible to fracture than less demanding anatomies. Stents or scaffolds for peripheral vessels such as the SFA, require a high degree of crush recovery. The term “crush recovery” is used to describe how the scaffold recovers from a pinch or crush load, while the term “crush resistance” is used to describe the force required to cause a permanent deformation of a scaffold. It has been believed that a requirement of a stent for SFA treatment is a radial strength high enough to maintain a vessel at an expanded diameter. A stent which combines such high radial strength, high crush recovery, and high resistance to fracture is a great challenge.